Autonomous vehicles, such as vehicles that do not require a human driver, can be used to aid in the transport of passengers or items from one location to another. Such vehicles may operate in a fully autonomous mode where passengers may provide some initial input, such as a pickup or destination location, and the vehicle maneuvers itself to that location by controlling various systems of the vehicles, including acceleration and deceleration (braking) systems of the vehicle.
As shown in the example prior art hydraulic braking system 100 of FIG. 1, a typical vehicle may include a split hydraulic brake system that includes multiple hydraulic circuits. As shown, a pair of hydraulic circuits are connected through a hydraulic fluid reservoir 10. Each hydraulic circuit includes a first or second chamber 20, 22. First chamber 20 is defined by a pair of seals 30, 32, and second chamber 22 is defined by seal cylinder wall 46, respectively. Once the vehicle's brake pedal is depressed, this cause a push rod 40 (which may also include a spring, not shown) to force a plunger 42 towards a floating piston 38 which increases the pressure in the first chamber 20 and the second chamber 22. Eventually, with enough force, the seals 30, 34 are moved towards the cylinder wall 46 and eventually cut off the hydraulic fluid reservoir 10 from the first and second chambers 20, 22 causing a the floating piston 38 to move even further towards the cylinder wall 46. This, in turn, causes the floating piston to force the seal 34 towards the end wall 46. Pressure in the two chambers 20, 22 is equalized through the floating piston 38 and springs 46, 48. Spring 46 functions to separate the seals 30, 32, and spring 48 provides resistance between seal 34 and end wall 46. As shown, the inlet traces 31, 35 from the reservoir 10 to the chambers 20, 22 are be very close to the seals 30, 34 in order to ensure that at rest, fluid can flow from the chambers to the reservoir but as soon as the plunger 42 moves to the right, the seals 30, 34 slide towards the end wall 46 and close off the flow path to the reservoir. Each one of the circuits may control a pair of brakes, for instance, front and rear, via lines 50, 52.
FIG. 2 depicts a chart 200 with example braking profiles relating the percentage of braking (how much deceleration is effected) given the distance that the brake pedal is moved towards a maximum position or maximum depression level. As can be seen by the normal deceleration curve (when there is no hydraulic system failure), for the most part, between about 10% and about 90% distance, the movement of the brake pedal has a generally linear relationship with the amount of deceleration.
Returning to FIG. 1, in the case of a total loss of hydraulic pressure or a slow leak in the hydraulic system, the hydraulic braking system 100 may allow a human driver the ability to still be able to brake (i.e. slow a vehicle down). For instance, via a leak in the line 50 to the front brakes, pressure may drop in the first chamber 20. In this case, a “normal” braking force would not build enough pressure from the floating piston 38 against the seal 34 to cause the front or rear brakes to engage properly. A human driver may instinctively continue to push on the brake pedal until the rod builds up enough pressure to force the floating piston 38 to move the seal 34 towards the end wall. This would build up pressure in the second chamber 22 and cause the hydraulic fluid to move via line 52 towards the rear brakes, thereby slowing the vehicle down. Returning to FIG. 2, as can be seen by the front hydraulic failure curve (when there is a failure in the line 50), there is no braking power until the brake pedal has been moved almost 30% of the total distance possible. At this point, the shape of the curve is somewhat similar to that of the normal deceleration curve.
However, because the first piston 20 has to be moved some distance which is significantly greater than that required under “normal” non-failure circumstances, the brakes would feel much less responsive. In other words, the maximum amount of braking may be reduced, and a deceleration that may normally have required a very slight pedal push may require a significantly harder push and a much further depression of the brake pedal. A similar situation would occur in the event of a leak in the line 52 that goes to the rear brakes as can be seen from the rear hydraulic failure curve of FIG. 2. While a human driver may automatically respond by continuing to press harder on the brake pedal until the vehicle begins to slow down to a desired level, autonomous vehicles do not have such capabilities and as discussed further below, may encounter additional problems when such failures occur.